The present invention relates to a system for producing a liquid crystal driving signal and an optical disc reproducing system provided with the system for producing a liquid crystal driving signal.
Recently, the DVD (Digital Versatile Disc) having a large capacity larger than that of the compact disc has been proposed. Since the DVD has a high density, the tilt angle margin of the disc at the reproducing thereof is small. Therefor, it is desirable to provided a control means for adjusting the tilt angle in the reproducing system.
The assignee of the present invention proposed a system for correcting the wavefront aberration by using a liquid crystal panel, the aberration is caused by the inclination of the thickness of the disc, in the Japanese Patent Application Laid Open Publication 9-128785.
FIG. 6 shows an optical disc reproducing system using the principle disclosed in the publication. A laser beam emitted from a laser 1 is reflected from a half mirror 2, passes through a liquid crystal panel 3 and applied to an optical disc 5 by an objective 4 to form a spot on the disc.
A laser beam reflected from the optical disc 5 passes through the objective 4, liquid crystal panel 3, and half mirror 2, and condensed on a photo-detector 7 by a condenser lens 6. Thus, a pickup is composed.
A tilt sensor 8 is provided adjacent the pickup for detecting the tilt angle of the disc 5. The tilt sensor 8 has one light emitter and two photo-detectors which receive light reflected from the disc 5. Outputs of the two photo-detectors are applied to an adder 10 which produces a difference of the outputs as a tilt error signal. The error signal is fed to an A/D converter 11.
A CPU 17 applies a control signal based on the error signal to a gain adjuster 13. The gain adjuster 13 adjusts the amplitude (gain) of a standard signal fed from an oscillator 12 in accordance with the control signal. The standard signal has, for example, a duty ratio of 50%. The adjusted standard signal is applied to the liquid crystal panel 3 as a driving signal. The gain adjuster 13 comprises a plurality of adjusting devices, each of which is connected to each unit electrode of the liquid crystal panel so as to adjust the tilt angle of the disc 5 as described hereinafter.
FIG. 7a is a sectional view of the liquid crystal panel 3, and FIG. 7b is a plan view of the panel. The liquid crystal panel 3 comprises opposite transparent glass substrates 301a and 301b, transparent electrodes 302a and 302b evaporated on the substrates 301a , 301b, orientation films 303a and 303b, and liquid crystal 303 sealed between the orientation films.
At least one of the electrodes 302a and 302b comprises a plurality of unit electrode 302c arranged in matrix as shown in FIG. 7b. Each of the unit electrode 302c is connected to a corresponding gain adjuster 13.
FIG. 8 shows a molecule M of the liquid crystal 304. The molecule M has a refractive index of n1 in the optical axis and a refractive index of n2 in the direction perpendicular to the optical axis. Namely, the liquid crystal is a double refraction crystal having different refractive indexes in direction. For example, the liquid crystal is nematic liquid crystal.
FIGS. 9a to 9c show various directions of molecules M when applied voltage is changed.
FIG. 9a shows the direction when no voltage is applied. FIGS. 9b and 9c shows directions when alternating voltages e1 and e2 are applied (e1&lt;e2).
As shown in figures, the direction of the molecule changes from the horizontal direction to the vertical direction.
Each of FIGS. 9a to 9c shows the state when equal voltages are applied to all unit electrodes of the liquid crystal 3 so that the directions of the molecules in each figure is the same. However, if the voltage applied to each unit electrode is different from that of other pieces, the direction necessarily changes. Thus, the refractive index of each molecule can be changed between n1 and n2.
When the refractive index of the liquid crystal is changed, the light beam passing through the liquid crystal changes in the optical path of the difference given by the following formula. EQU Optical path difference: .DELTA.n.multidot.d
where .DELTA.n is the change quantity of the refractive index and d is the thickness of the liquid crystal.
This means that a phase difference obtained by the following formula is given to the light beam passing through the liquid crystal. EQU Phase difference: .DELTA.n.multidot.d(2.pi./.lambda.)
where .lambda. is the wavelength of the light beam.
Therefore, it is possible to correct the wavefront aberration caused by the inclination of the disc by controlling the refractive index n of each part of the liquid crystal so as to cancel the aberration generating in the objective 4.
However, in the liquid crystal driving signal producing system, it is a problem to realize the controlling of the amplitude of the liquid crystal driving signal.